Presently, in-cylinder injection type internal combustion engines, which inject fuel directly into the combustion chamber, have been put to practical use. In such in-cylinder injection type internal combustion engines, since timing to inject fuel can be set freely, fuel injection is performed on a compression stroke in a low load operating area, a mixture with fuel concentration sufficient for ignition is collected locally near a spark plug, and super lean combustion by so-called stratified-charge combustion is performed, whereby an even further enhancement in fuel consumption is contrived.
In an in-cylinder injection type internal combustion engine as described above, the operation in this super lean region is performed in a predetermined operating region/so it is difficult from the side of exhaust purification to make the exhaust gas characteristics satisfactory by providing only a three-way catalyst (having a three-way function near a stoichiometric ratio) which is used in a multipoint injection (MPI) engine, etc.
Hence, a lean NO.sub.x catalyst that can purify NO.sub.x even in an excess oxygen concentration condition, in which oxygen in exhaust gases becomes excessive, has been developed, and providing this NO.sub.x catalyst is indispensable.
For this lean NO.sub.x catalyst, types which purify NO.sub.x in exhaust gases by adhering NO.sub.x to a catalyst (an occlusion type lean NO.sub.x catalyst and a trap type lean NO.sub.x catalyst) have been developed.
This lean NO.sub.x catalyst has the function of adhering NO.sub.x in exhaust gases to itself in an excess oxygen concentration condition and desorbing the adhered NO.sub.x if oxygen concentration is reduced. In other words, in an oxygen excess concentration condition, the lean NO.sub.x catalyst has a function of oxidizing NO in exhaust gases and generating a nitride, thereby adhering NO.sub.x to itself. On the other hand, in a condition in which oxygen concentration has been reduced, the lean NO.sub.x catalyst has the function of causing the nitride adhered to itself and CO in exhaust gases to react and generating a carbonate, thereby desorbing NO.sub.x.
Incidentally, fuel or lubricating oil contains a sulfur component (S component), and such a sulfur component is also contained in exhaust gases. In the lean NO.sub.x catalyst, in an excess oxygen concentration condition, NO.sub.x adheres and also the sulfur component adheres. In other words, the sulfur component burns, and furthermore, it is oxidized on the lean NO.sub.x catalyst into SO.sub.3. And part of this SO.sub.3 further reacts with an NO.sub.x occluding agent on the lean NO.sub.x catalyst and becomes a sulfate, so that it adheres to the lean NO.sub.x catalyst.
Therefore, a nitride and a sulfate adhere to the lean NO.sub.x catalyst, but, since the sulfate is higher in stability as a salt than the nitride and only a portion thereof is resolved even in a condition in which oxygen concentration has been reduced, the quantity of the sulfate remaining on the lean NO.sub.x catalyst increases with time. With this, as the NO.sub.x adhesion ability of the lean NO.sub.x catalyst is reduced with time, the catalytic performance of the lean NO.sub.x catalyst is degraded (this is referred to as S poisoning).
The nitride which reduces the NO.sub.x adhesion ability of such a lean NO.sub.x catalyst has the resolve property if temperature becomes high.
For this reason, for example, in a technique disclosed in Japanese Laid-Open Patent Publication No. SHO 63-150441, the opening angle of an idle speed control valve is increased to increase an intake air quantity and the engine revolution speed is raised to a high revolution region (2000 to 3000 rpm) and maintained, whereby an attempt is made to hold the catalyst bed temperature at high temperatures. And in this state, the quantity of fuel is increased to make an air-fuel ratio rich, whereby an attempt is made to cause exhaust gases passing on a catalyst to be in a condition of deoxidization.
However, since a special operating state such as this will have influence on the output torque of an engine, it cannot be applied to any engine. For example, in the case where such a technique is applied to engines for automobiles, if the above-mentioned special operating state is produced during the normal operation of an automobile, it will have influence on the travel of the automobile and be difficult to put to practical use.
In addition, for example, in a technique disclosed in Japanese Laid-Open Patent Publication No. HEI 6-66129, when a certain quantity or greater of sulfur component adheres to a lean NO.sub.x catalyst, the air-fuel ratio of exhaust gases is made a stoichiometric air-fuel ratio or made rich, and the exhaust gases are heated and raised in temperature by an electric heater arranged around an exhaust pipe, whereby an attempt is made to resolve and desorb the sulfur component from the lean NO.sub.x catalyst.
However, in this technique, there is a need to arrange an electric heater to raise the temperature of exhaust gases, so that the cost is considerably increased. On the other hand, in the electric heater, warm-up time is required and it takes time to raise the temperature of exhaust gases, so that it is difficult to regenerate the purification efficiency of the catalyst early. In addition, it is undesirable to make an air-fuel ratio a stoichiometric air-fuel ratio or make it rich, because if done, fluctuation will occur in the engine output torque.
The present invention has been made in view of such problems, and it is an object of the invention to provide an exhaust purifying apparatus for an in-cylinder injection type internal combustion engine which is capable of reliably desorbing a sulfur component adhered to an NO.sub.x catalyst and enhancing the durability of the NO.sub.x catalyst, by reliably raising exhaust gas temperature, while providing no additional device and furthermore having no influence on an engine output torque.